Threaded connectors or couplings for use in connecting adjacent sections of a tubing string are well known. In the oil and gas industry, threaded connectors have long been used in a wide variety of applications, such as connecting adjacent sections of a well casing, a drill string, or a pipeline.
Typically, a threaded connector is designed to resist static loads and to provide a reliable, pressure-tight seal. The static load applied to a threaded connector may be very large, as in a well casing or a drill string where the connector may be required to support the weight of as much as a thousand feet or more of heavy steel tubing. Further, a well casing may be used to produce caustic or corrosive fluids which are at extremely high temperatures and pressures. The threaded connectors used in such a well casing must prevent leakage of these fluids into low pressure strata and fresh water sands traversed by the wellbore.
In other applications, a threaded connector must also be able to withstand high-cycle dynamic variations in tensile and bending stresses. As is well known, these high-cycle dynamic stress variations can result in a fatigue failure of the connector. As more fully described below, examples of this type of connector are the threaded connectors used in a floating drilling riser or a tension-leg-platform tether.
In offshore floating drilling operations, a riser is used to guide the drill string into the subsea well and to provide a path for conducting the drilling fluid back to the vessel. The riser is connected at its lower end to the subsea wellhead and at its upper end to the floating drilling vessel. The weight of the riser and the drilling fluid contained therein is typically supported by large pneumatic or pneumatic/hydraulic tensioners located on the floating drilling vessel. These tensioners also compensate for vertical heave of the vessel due to the action of waves and tides.
The connectors used to connect adjacent sections of a floating drilling riser are subject to substantial variations in tensile and bending stresses resulting from lateral movements of the drilling vessel induced by wind, waves, and currents. Furthermore, in some areas of the world, such as offshore Brazil, the riser is subject to very high subsea currents which tend to cause it to vibrate laterally. The connectors used in the riser must be able to withstand these cyclic stress variations without fatigue failure.
A tension-leg-platform (hereinafter referred to as a "TLP") is an offshore structure designed to have a compliant rather than a rigid response to environmental forces such as wind, waves, and currents. Typically, a TLP has a buoyant main body (the "hull") which floats on the surface of the body of water and supports the drilling rigs and other equipment used in petroleum drilling and producing operations. The hull is secured to a foundation on the floor of the body of water by a set of substantially vertical tendons or tethers. Each tether typically comprises a series of elongated tubing sections connected by threaded or bolted connectors. The length of the tethers is carefully adjusted to ensure that the hull is maintained at a somewhat greater draft than would be the case were it unrestrained. The resulting excess buoyancy of the hull exerts an upward load on the tethers, maintaining them in tension. The tensioned tethers restrict pitch, roll, and heave of the hull in response to environmental forces while maintaining the hull in position above the subsea wells. However, the environmental forces acting on the hull result in cyclic variations in tensile and bending stresses in the tethers. The connectors used to connect adjacent sections of the tethers must be capable of withstanding these cyclic stress variations without fatigue failure for the anticipated lifetime of the TLP, which may be as long as twenty years or more.
One method for avoiding fatigue failures in a threaded connector is to use a massive connector. An example of this method is illustrated in U.S. Pat. No. 4,708,513 issued Nov. 24, 1987 to Roche et al. Use of a massive connector results in a decrease in the average stress levels in the connector and, accordingly, an increase in the expected fatigue life of the connector. However, this method results in an inefficient solution to the fatigue problem. Further, the excess weight of the connector can be a problem in situations where the overall weight of the tubing string is critical, such as a TLP tether or a floating drilling riser.
Accordingly, there is a need for a low weight, fatigue-resistant threaded connector for use in a tubing string which is subject to cyclic stress variations.